There's too many PC games to say I have a favourite. I think Half-Life left one of the biggest impressions on me.
However I have preferences for different reasons. Serious Sam 3 has almost endless replayability, and I have clocked 220 hours in campaign. Cyrsis 3 has great graphics. Alien Rage - Unlimited had sci-fi oozing out of it. Alien: Isolation was like an Alien/s film set, and has incredible graphics. Although I can't pin down why, I love teh Zommie Army games.
I love the 90's computer game "Street Wars" it is some kind of a gangster simulation with building an little empire. At a time I was really addicted, but haven't played it in a while because the cd doesn't work on my laptop (to old )